The invention relates to a process and equipment for filtering liquids, especially water flowing horizontally through a sand filter grain column placed in a tower filter, where the regeneration and cleaning of the contaminated filter parts is carried out exclusively with the injection of liquid jets.
Filtering of liquids has become a frequent practice during the recent decades in various fields of industry, and this process has gained vital importance due to the increasing water shortage consequent upon the rapidly increasing water consumption. As a result, tower filters stepped into the foreground, representing great progress in the filtering of liquids, carrying out the filtering with the liquid flowing horizontally through the vertical filter tower, and which--in contrast with the earlier horizontal filtering methods and equipment--have realized the more economical cleaning of the untreated waters, as well as a reduction of investment and operating costs and an improvement of space utilization.
Such process and equipment are described in Hungarian Pat. No. 154,412. Also the invention described in the Hungarian Pat. No. 163,094, represents mainly the improvement achieved in the quality of filtering compared to that achieved by the earlier tower filters.
The experiences acquired in the course of filter tower operations brought up certain difficulties and disadvantages, on the basis of which it became possible to determine the direction of further development. The difficulties turned up first of all in connection with the applied method according to which regeneration of the contaminated filter column was carried out in all types of towers by recirculation or backwashing after a long- or short term operation in such a way, that the contaminated filter grains were carried from the cone-shaped collecting space at the bottom of the filter tower into the collecting space on the top of the filter through pipes provided for this purpose, placed outside or inside the filter tower by compressed air flow, injector, mammoth-pump. Then after washing and settling the filter grains were carried or washed back into the upper part of the filter column. These known methods of regeneration are unfavourable in several respects, such as:
dead spaces remain in the process of backwashing, deteriorating the quality of filtering due to slump of the sand, PA1 the pipe system and machinery for the recirculation incur additional costs in regard to investment and maintenance, PA1 owing to clogging of the narrow cross sections of the slurry pipes, the presently used methods of recirculation results in considerable loss of operating time and loss of capacity, PA1 recovery of the filter grains from the cone-shaped collecting tank during regeneration restricts the conical formation of the tower filter, for it requires significant inactive height, PA1 the conical bottom is especially unfavourable in the high-capacity filter units since it further reduces the proportion of useful volume. Additionally, the cost of tooling and for statical reasons the specific material utilization also increases.
Cleaning the periodically clogged up sieve surfaces that form the walls of the filtered water collecting space similarly causes great difficulty in the known tower filters, because the large surface reduces the speed, consequently the cleaning with back flushing can not be carried out effectively and in acceptable quality.
Finally it is difficult to remove the sand from the untreated water-distributing space developed differently in the various types of tower filters, into which the contaminated sand passes mainly during regeneration of the filter grain column.
The invention is aimed at eliminating the existing disadvantages occurring in regeneration of the tower filters, and attaining a more effective, better, cheaper and more productive solution for the regeneration and cleaning of the contaminated filter grain column and other filter parts.
A further object is to increase the economic efficiency through a reduced space requirement and use of fewer raw materials.
The invention is based on the recognition that the above goal is attainable, thus the disadvantages connected with costliness of the equipment, quality of the filtering and time of the regeneration arising mainly due to regeneration of the tower filters can be completely eliminated, provided that regeneration is carried out exclusively with injection of water jets of proper position, direction and speed.
Thus the invention is a process for filtering liquids flowing horizontally through the filter column, characterized in that the regeneration of the contaminated filter column is carried out without any extra equipment /mammoth-pump, compressor, or water jet pump/ only by injection of water jets in such a way, that the filter column is slurried at the bottom, bringing about reversed flow with pressure of the water jets, thus bringing about recirculation and backwashing, whereby the filter grains are cleaned. Since slurrying the bottom of the filter column can be extended to a smaller or larger area by varying the intensity of the injected water jets, hence the ratio between the recirculation and backwashing can be varied at will in the sand column. Backwashing of the filtered water collecting pipe and removal of the sand from the untreated water-distributing gap with water jet represent essential parts of the invention, the latter operations being applicable independently too in the earlier sand filters, or these filters can be converted subsequently as well.
The invention relates also to an equipment to carry out the process, in which according to the direction of the flow a narrow gap is brought about before the filter grain column with wall of nonoverlapping panels, communicating through a large gap underneath with the filter grain column. This gap serves for distribution of the water to be filtered and for the path of the filter grain recirculation during regeneration of the filter column. Furthermore a lower space is developed with deflecting plate in the equipment under the untreated water distributing gap, in order that after cleaning of the filter grain column, one or several water jets are injected for removal of the sand from the distributing gap, when by breaking up on impact and scattering the jets come into a swirling motion, then flowing upwards carry the filter grains getting here during the recirculation onto the top of the filter column, thereby freeing the untreated water distributing gap. Finally following the flow of liquid, a vertical collecting pipe--not letting through the sand grains, but letting through the filtered water--is placed into the equipment to the wall of the filter tank behind the filter column, for discharging the filtered water from the tank. The upper end of the collecting pipe is provided with a speed accelerating throat connected with the filtered water-return pipe.
The experiences acquired with the known tower filters have shown that even the many times oversized clean water-discharging surfaces can not prevent clogging of the sieve surface sooner or later. Such clogging may be caused for instance by the tiny grains always existing in the filter sand, wedged up in the sieve holes; the colloidal impurities passing through the sand layer get deposited on the sieve surfaces during temporary failure in the chemicals' supply; scale precipitating on the sieve, due to pressure drop in case of filtering deep waters. Such clogged up large surfaces can not be effectively cleaned with back flushing, because of the low velocity.
The solution to these problems according to the invention starts out from conceptual considerations contrary to the established practice. Namely from the idea that it uses such small surface only for collection of the filtered water, which just lets through the appropriate quantity of filtered water. Such relatively small surface can be back flushed effectively, because the backflushing water flows with sufficient velocity from the sieve surface to wash out the wedged up sand grains and adhering impurities. However, for this purpose it was necessary to ensure that the backwashing water should flow with adequate velocity both on the lower and upper end of the sieve surface. The experiments demonstrate that it is practicable to use a single vertical collecting pipe for collection of the filtered water, into which the flushing water is conducted from the top downwards through the speed accelerating throat at the rate of 5-15 m/sec.
The experiments prove, that in such case the flushing water does not escape through the upper part of the sieve pipe, because the injected water jet penetrates to the bottom of the sieve pipe, there impacting and congesting ensures a practically uniform flow-out on the whole surface.
In accordance with above, the regeneration is started at the end of each filtering period, by letting filtered water into the collecting pipe of the clean water from the top downwards at the already mentioned rate of 5-15 m/sec for a few seconds. Since according to above, only a single collecting pipe is used, the flushing water flows at a high speed on its relatively small surface into the filter column, and thus it cleans not only the jacket of the collecting pipe, but it washes out completely the thin sand layer in its full length between the collecting pipe and filter tank. This way formation of the slumping sand layer behind the collecting pipe becomes impossible.
This is followed with regeneration of the filter column. This is carried out by underwashing the filter grain column with water jets, and the slurry flowing in the opposite direction--making it to flow upwards in the untreated water distributing gap, that is connected with the sand column at the bottom--is carried to the top of the filter grain column, settled, and meanwhile the waste water is discharged through the overflow.
By the time the filter grain column is cleaned, the recirculating and backwashing water jets are turned off and conducted into the lower part of the untreated water-distributing gap in such a way, that the suitably distributed sand-removing water jets slurry the sand only in the untreated water distributing gap and wash it up onto the top of the sand column, without underwashing the filter column.
Velocity of the liquid to be filtered--conducted horizontally through the sand column in the filtering process of the invention--and the sand grain size are selected in such a way, that penetration of the filtered contaminating particles should extend only to about half of the filter column, while the other half of the column should serve as a protective layer to ensure the cleanliness of the filtered water.
Thus in one of the suggested constructional embodiments of the equipment according to the invention, the filter column is divided vertically with a perforated plate containing large holes, and this way the sand layer in the front and rear on both sides of the perforated plate can be regenerated separately from each other--the layer in the rear with less frequency--by slurrying, recirculation and backwashing, because the sand grains do not mix with each other in the practice, even if the holes are much bigger than the sand grains.
Since practically only the front layer of the filter column divided by the perforated plate gets clogged up during filtering, only this has to be recirculated during each regeneration, while the rear layer--which hardly gets contaminated--should be cleaned only occasionally. Thus by dividing the filter column into two layers with the perforated plate, the time of regeneration can be reduced to half in practice.
In another preferable construction alternative of the invention, two sand columns are placed in the filter tank symmetrically with the untreated water-distributing gap of common diametral plane, when the two filter columns are provided with separate collecting pipes.